1. Field of the Invention
The present invention relates to an original reading lens, an original reading lens unit, an original reading device, and an image forming device.
2. Related Art Statement
In an image forming device such as a facsimile and digital copier, an original image to be read is reduced by an image reading lens, and is imaged onto a solid-state image sensing device such as a CCD, and then image information is converted into signals. When an image is read by full range of colors, by using a so-called three line CCD in which light-sensitive elements having filters of red, green, and blue, for example, are arranged in three lines in one chip, an original image is imaged on its light sensitive surface, and is separated into three primary colors, and then the color image is converted into signals.
It is necessary for an image reading lens to have high contrast equally from proximity of an optical axis to its periphery in high spatial frequency region area. Accordingly, the field curvature of the image reading lens should be well corrected, and then the field curvature in a sagittal direction should be well corrected and coma flare should be controlled in order to equalize the contrast especially in a sub scanning direction.
In addition, in order to favorably read a color image, it is necessary for an imaging position of each color light such as red, green, and blue to be aligned in the direction of the optical axis on the light sensitive surface of the solid-sate image sensing device. It is also necessary to perform extremely favorable corrections of chromatic aberration and secondary spectrum (axis chromatic aberration of another wavelength when correcting axis chromatic aberration with respect to secondary wavelength) of each color. If the secondary spectrum is not fully corrected, resolution of reading with respect to an image containing large wavelength component of the secondary spectrum is decreased.
It is also required for the image reading lens that the vignetting factor is substantially 100% up to the peripheral part of the field angle.
In other words, in a color image reading lens for preferably reading a color image, curvature of field, axis chromatic aberration, and its secondary spectrum should be well corrected in terms of the aberrations.
In order to favorably correct the filed curvature, it is necessary to reduce Petzval sum as much as possible.
In order to preferably correct the axis chromatic aberration, it is favorable for a positive lens among the lenses constructing a color image reading lens to use a glass material having high refractive index and (large Abbe number) low dispersion and it is preferable for a negative lens among the lenses constructing a color image reading lens to use a material having low refractive index and (small Abbe number) high dispersion.
In order to favorably reduce the secondary spectrum of chromatic aberration, it is preferable for deviation of partial dispersion of each material from a reference line to be positive in a positive lens and to be negative in a negative lens.
However, among optical glasses which are mass-produced and easily obtained, lanthanum crown material and tantalum crown material having high refractive index and low dispersion, which are frequently used as a positive lens, are that the deviation of partial dispersion from a reference line is negative, and heavy flint material having low refractive index and high dispersion, which is frequently used as a negative lens is that the deviation of partial dispersion from a reference line is positive.
Accordingly, in case of using the above easily obtained optical glass, it is extremely difficult to preferably correct axis chromatic aberration while reducing Petzval sum, and also to favorably correct the secondary spectrum of the axis chromatic aberration.
Conventionally, there has been known a color image reading lens using Gauss type of a four-group, six-element lens in which the field curvature is favorably corrected up to about 20° of the half field angle, the generation of coma flare can be controlled even though the aperture is relatively large, and the correction ability of chromatic aberration is high. Such a conventional color image reading lens uses a so-called abnormal dispersion glass in which the deviation of the partial dispersion from the reference line is negative is used for at lease one of the negative third lens and the negative fourth lens for correcting the secondary spectrum of the axis chromatic aberration. (Reference to JP2729039 and JP2790919, JP-A-H09-304696, H10-253881, H11-109221, and 2001-166359.)
However, such a Gauss type is composed of 6 lenses, which is a large number of lenses, so that the outer diameter of the lenses tends to be large, and it is difficult for the color image reading lens to be downsized. Therefore, an image reading device and image forming device tend to be bigger.
The abnormal dispersion glass having about 40 to 45 of Abbe number used for the above negative lens has a processing problem which causes white turbidity (so-called tarnish) on a lens surface by the action of adhesive and cleaning medical agent used for grinding or cleaning. Consequently, because of the tarnish on a lens surface, it is hard to realize high yield, and the processing cost is easy to increase.
Moreover, recycling of processing material is recently requested in various fields of industry with the object of resource saving, and the recycling of the glass material for lens is also intended. Seen from the recycling of the glass material for lens, it is obvious that harmful substance is not contained in the lens material.